G protein-coupled receptors, also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors, constitute a large protein family of receptors that sense molecules outside the cell and activate inside signal transduction pathways and, ultimately, cellular responses. GPCRs can be categorized into six classes based on sequence homology and functional similarity: Class A (Rhodopsin-like), Class B (Secretin receptor family), Class C (Metabotropic glutamate/pheromone), Class D (Fungal mating pheromone receptors), Class E (Cyclic AMP receptors), and Class F (Frizzled/Smoothened).
G-protein coupled receptor family C group 5 member D (GPRC5D) is an orphan receptor with no known ligand or function in humans. It is a member of a family of retinoic acid-inducible G-protein-coupled receptors. It is overexpressed in multiple myeloma (MM) cells and is not expressed or expressed in a significantly lower level by any other cell type, benign or malignant, as shown in FIG. 1. Several groups have identified this gene as highly differentially expressed by gene expression profiling of primary MM cells when compared to normal tissue1 or other hematologic malignancies (Frigyesi, I., et al. Robust isolation of malignant plasma cells in multiple myeloma. Blood 123, 1336-1340 (2014); Cohen, Y., Gutwein, O., Garach-Jehoshua, O., Bar-Haim, A. & Kornberg, A. GPRC5D is a promising marker for monitoring the tumor load and to target multiple myeloma cells. Hematology (Amsterdam, Netherlands) 18, 348-351 (2013); Bam, R., et al. GPRC5D Is a Cell Surface Plasma Cell Marker Whose Expression Is High In Myeloma Cells and Reduced Following Coculture With Osteoclasts. Blood 122, 3099 (2013)). It has been shown that higher mRNA expression correlates with worse overall survival (Atamaniuk, J., et al. Overexpression of G protein-coupled receptor 5D in the bone marrow is associated with poor prognosis in patients with multiple myeloma. European journal of clinical investigation 42, 953-960 (2012)). Surface staining of Bone marrow aspirates from patients with MM demonstrate plasma cell specific staining (B am, R., et al. GPRC5D Is a Cell Surface Plasma Cell Marker Whose Expression Is High In Myeloma Cells and Reduced Following Coculture With Osteoclasts. Blood 122, 3099 (2013)). Given the significant role for GPRC5D in multiple myeloma, antibodies that recognize GPRC5D, and methods of using such agents, are desired.